diff --git a/lib/matplotlib/colors.py b/lib/matplotlib/colors.py
index 35550f0b743..511f46d1871 100644
--- a/lib/matplotlib/colors.py
+++ b/lib/matplotlib/colors.py
@@ -925,7 +925,9 @@ def __call__(self, value, clip=None):
                 result = np.ma.array(np.clip(result.filled(vmax), vmin, vmax),
                                      mask=mask)
             # ma division is very slow; we can take a shortcut
-            resdat = result.data
+            # use np.asarray so data passed in as an ndarray subclass are
+            # interpreted as an ndarray. See issue #6622.
+            resdat = np.asarray(result.data)
             resdat -= vmin
             resdat /= (vmax - vmin)
             result = np.ma.array(resdat, mask=result.mask, copy=False)